At part load conditions, Francis turbines are subject to the emergence of a hydrodynamic instability in their draft tube, referred to as precessing vortex rope. It induces pressure pulsations in the ...water passages at the precession frequency of the vortex, leading to additional vibrations and dynamic loads on the runner blades. The prediction of both the dynamic behaviour of the vortex rope and the resulting dynamic loads over a wide operating range is of importance to improve the runner design and robustness on the one hand and to assess additional fatigue and related maintenance costs on the other hand. Such a prediction, either with numerical simulation or reduced scale physical model tests, remains however challenging. The present paper aims at introducing a methodology to assess the vortex behaviour, the related pressure fluctuations and the resulting dynamic strains on the runner over the complete part load operating range. It is based on reduced scale physical model tests of a Francis turbine, including the measurement of the pressure and the load on the runner with instrumented blades. It is shown that the influence of both the discharge factor and speed factor on the vortex dynamics behaviour and related pressure fluctuations can be represented by a single parameter; the swirl number. The correlation with the swirl number is further extended to the dynamic strains induced by the vortex rope on the runner blades. Similar mechanical load and pressure measurements are finally performed on the full-scale machine during a power ramp-up and the results are compared to the empirical correlations established on the reduced scale physical model.
Nanocomposites fabricated based on a compound of carbon nanofiller and polymer matrix exhibit extraordinary piezoresistive performance, and the capacity of nanocomposite sensors in measuring ...structural dynamic strains has been widely demonstrated. However, the sensitivity of existing nanocomposite strain sensors is limited, particularly in aspects of sensing precision and response frequency. This study focuses on sensitivity investigations of flexible nanocomposite sensors fabricated using a compound of carbon black (CB) fillers and polyvinylidene fluoride (PVDF) matrix, which has shown promising potential in perceiving extremely weak strain signals within a considerably broad range of response frequencies.
As a highly important indication of sensitivity, sensing precision was characterized in a quantitative manner. Broadband spectrum analysis was conducted as an effective way to examine sensor performance in perceiving minimal disturbances in dynamic strains. Disturbances were generated in two experiments, by introducing local material and geometric variations into the inspected structures, in terms of mass attachment and structural damage. The effectiveness of the nanocomposites in structural damage detection was demonstrated. Moreover, the experimental results indicate the promising potential of nanocomposite-based strain sensors for applications such as advanced bioelectronics, ultrasonic inspection, and in-situ structural health monitoring.
The aim of this paper was to explore the application of multi-channel synchronized dynamic strain gauges in monitoring the neutral axis (N.A.) position of prestressed concrete box girders. The N.A. ...position has recently been proposed as an indicator for monitoring the health of bridge structures. Laboratory experiments were conducted on a prestressed T-beam under different prestress level conditions to investigate the correlation between the prestress magnitude and the N.A. position. In the development of the multi-channel synchronized dynamic strain gauges, edge computing was employed to significantly reduce the amount of data transmitted from the sensor nodes on-site. In edge computing, only the dynamic strain response caused by the maximum vehicle load in each minute is transmitted. This approach greatly enhances the monitoring efficiency and enables the realization of on-site non-computer-based monitoring systems. The laboratory test results of the prestressed T-beam showed that the N.A. position tends to move slightly downward as the prestress force increases. In other words, when the prestress force decreases due to loss, the N.A. position exhibits a slight upward movement. This study selected a newly constructed prestressed box girder as the subject for on-site measurement of the N.A. position using multi-channel synchronized dynamic strain gauges shortly after the prestress was applied. The on-site monitoring data indeed revealed a gradual upward movement of the N.A. position. This phenomenon confirmed that soon after the completion of prestressed concrete bridges, there is a gradual loss of prestress due to the significant shrinkage and creep effects of the early-age concrete. The on-site monitoring result aligned with the findings from the laboratory experiments, where the N.A. position was observed to move upward as the prestress decreased.
This study proposes an innovative method for structural health monitoring of simply supported PCI girder bridges based on dynamic strain and edge computing. Field static and dynamic load tests were ...conducted on a bridge consisting of a span with newly replaced PCI girders and numerous spans with old PCI girders. Both the static and dynamic test results showed that the flexural rigidity of the old PCI girders decreased significantly due to deterioration. To improve the efficiency of on-site monitoring data transmission and data analysis, this study developed a smart dynamic strain gauge node with the function of edge computing. Continuous data with a sampling frequency of 100 Hz were computed at the sensor node. Among the computed results, only the maximum dynamic strain data caused by the passage of the heaviest vehicle within 1 min were transmitted. The on-site monitoring results indicated that under routine traffic conditions, the dynamic strain response of the new PCI girder was smaller than that of the deteriorated PCI girder. When the monitored dynamic strain response has a tendency to magnify, attention should be paid to the potential prestress loss or other deterioration behaviors of the bridge.
The impact wave response of soil due to a ball drop is monitored on a 30.5 cm by 30.5 cm square soil box using a fiber sensor with dynamic strain sensing capability. The experiments are conducted in ...real time using a simple one-laser one-modulator configuration with stimulated Brillouin scattering. The embedded BOTDA sensor grid successfully monitored the distribution and evolution of the inner strains of a sand bed during a mass impact on its surface. The measurement of the distributed dynamic strains was possible in several milliseconds and with 1 cm actual location resolution. This paper presents a time-domain signal analysis utilized for determining the dynamic strains in embedded fiber sensor. The results demonstrate the method to be a promising one for detection of subsurface vibration and movement in geotechnical Structure Health Monitoring (SHM).
Basic approaches are outlined to solving complex engineering problems arising in the design and operation of power units at nuclear power stations (NPS) and based on large-scale physical simulation ...of dynamic conditions and parameters of dynamics and structural strength of modern power facilities. A procedure for physical simulation of specific dynamic processes in hydroelastic systems has been developed. Taking into account the identified distributions of pressure pulsations and flow rates on the surfaces of the structures of the investigated reactor plants (RU), hydrodynamic loads in various structures and facilities of modern power-machine buildings were determined considering the found distribution of fluctuations in flow pressure and velocity acting on the structure surfaces. The dynamic response parameters of the main structural members, such as dynamic strains and stresses, were obtained and analyzed. The effect of added masses of liquid, damping, and other factors on the dynamic processes in hydroelastic systems and ways for its reduction to increase the durability and service life of structures, first of all the critical elements of the studied power systems for each commissioned reactor, are determined. This approach is based on a comprehensive investigation of the interaction of a turbulent flow with the multicomponent structure of a reactor unit. The results of investigation confirmed the structure serviceability as to the relative error. The fitness for service of a structure during a specified operation time of the power unit was assessed. The implementation of the proposed approach involving large-scale physical simulation and, in many cases, numerical modeling, confirmed its effectiveness and the validity of the results obtained by the authors.
Sweden has a wide network of gravel roads. When the water table levels rise during the freeze-thaw and wet seasons, the bearing capacity of the roads will decrease and rut depths increase. The road ...will suffer a small deterioration every time a vehicle travel over it. Therefore, it has been decided to investigate the long-term performance of a Swedish designed gravel road using an accelerated pavement testing facility.
A trial gravel road was constructed and exposed to various levels of ground water table which simulate the weather changes in Sweden. The structure of the trial gravel road consisted of 15cm (base and surface layer), 20cm sub-base layer and 260cm subgrade soil. The trial road was tested using the Heavy Vehicle Simulator (HVS) and the vertical surface deformations and the vertical dynamic deformations were measured by means of laser profile measurement and εMU coils respectively. At the end of 85000HVS passes, the tested road suffered from serious deformations as discussed in details in this paper. Furthermore, a transverse trench section was excavated at the end of HVS trafficking in order to investigate the possible mechanism contributing to the tested road failure.
We analyze the ground motion time histories due to the local seismicity near the Itoiz reservoir to estimate the near-source, surface 3D displacement gradients and dynamic deformations. The seismic ...data were obtained by a semipermanent broadband and accelerometric network located on surface and at underground sites. The dynamic deformation field was calculated by two different methodologies: first, by the seismo-geodetic method using the data from a three-station microarray located close to the dam, and second, by single station estimates of the displacement gradients. The dynamic deformations obtained from both methods were compared and analyzed in the context of the local free-field effects. The shallow 1D velocity structure was estimated from the seismic data by modeling the body wave travel times. Time histories obtained from both methods result quite similar in the time window of body wave arrivals. The strain misfits between methods vary from 1.4 to 35.0 % and rotational misfits vary from 2.5 to 36.0 %. Amplitudes of displacement gradients vary in the range of 10
−8
to 10
−7
strains. From these results, a new scaling analysis by numerical modeling is proposed in order to estimate the peak dynamic deformations for different magnitudes, up to the expected maximum
M
w
in the region (M5.5). Peak dynamic deformations due to local
M
w
5.5 earthquakes would reach amplitudes of 10
−5
strain and 10
−3
radians at the Itoiz dam. The single station method shows to be an adequate option for the analysis of local seismicity, where few three-component stations are available. The results obtained here could help to extend the applicability of these methodologies to other sites of engineering interest.
The 11/5/2011 Lorca, Spain earthquake (Mw
=
5.2) and related seismicity produced extensive damage in the town of Lorca and vicinity. During these earthquakes, evidence of rotations and permanent ...deformations in structures were observed. To analyze these aspects and study the source properties from the near-field, the displacement time histories were obtained including the static component at Lorca station. Displacement time histories were computed by an appropriate double time integration procedure of accelerograms. Using these data, the foreshock and mainshock slip distributions were calculated by means of a complete waveform kinematic inversion. To study the dynamic deformations, the 3D tensor of displacement gradients at Lorca station was first estimated by a single station method. Using the finite fault inversion results and by means of a first order finite difference approach, the dynamic deformations tensor at surface was calculated at the recording site. In order to estimate the distribution of the peak dynamic deformations, the calculation was extended to the close neighboring area of the town. The possible influence of the near-field deformations on the surface structures was analyzed.
PDF
